1. Field of the Invention
The present invention relates to a cleaning process for cleaning an etching chamber of an etching machine. More particularly, the present invention relates to a waferless seasoning process to avoid the first wafer effect.
2. Description of the Related Art
In the process for manufacturing the gate electrode of a transistor, an anisotropic etching step is a common method used to pattern the gate electrode. In the anisotropic etching step, particles with high energy in a plasma are used to bombard and etch the conductive layer comprising a tungsten silicon layer and a polysilicon layer, so that some deposition, such as particles or polymer, accumulate on the inner wall of the etching chamber. Because the plasma bombards the deposit during performance of the anisotropic etching step and the deposit may fall on the wafer during the etching process, a critical dimension bias (CD bias) of the devices occurs.
When the deposit is thick enough to affect the etching process after the etching process is performed many times or when the etching environment in the etching chamber changes after the etching machine is idle for more than one hour, it is necessary to restore the etching chamber by a seasoning process. The seasoning process is a pre-treatment of the etching process.
Conventionally, the seasoning process comprises the steps of a dry cleaning process and an etching-chamber recovery process. The step of the dry cleaning process comprises the steps of placing three wafers in the etching chamber after the radio frequency (RF) electrical power supplier of the etching chamber operates for 500 minutes and then cleaning the etching chamber by a plasma for 5 minutes to remove the deposit on the inner wall of the etching chamber, wherein the plasma is a mixed gas comprising chlorine (Cl2) and sulfonium hexafluoride (SF6).
Thereafter, the etching-chamber recovery process is performed. Since the most of the deposit in the etching chamber is removed by the dry cleaning process, the etching environment in the etching chamber is greatly changed and the etching result on the conductive layer is unstable. In order to attain the most stable devices possible, it is necessary to perform an etching-chamber recovery process to stabilize the etching environment in the etching chamber.
Typically, the etching-chamber recovery process comprises the steps of placing several dummy wafers in the etching chamber and then performing an etching-chamber recovery process by the plasma used in a normal etching process. The etching process for etching a conductive layer comprises three steps of etching a photoresist, a tungsten silicide layer and a polysilicon layer. Consequently, in the etching-chamber recovery process, the dummy wafers are first bombarded by a plasma for 90 seconds, wherein the plasma is formed by a mixed gas comprising oxygen and hydrogen bromide (HBr). Second, the dummy wafers are bombarded by a plasma for 140 seconds, wherein the plasma is formed by a mixed gas comprising Cl2 and HBr. Finally, the dummy wafers are bombarded by a plasma for 100 seconds, wherein the plasma is formed by a mixed gas comprising HBr, helium and HeO2. In the etching-chamber recovery process, it is necessary to use 3-10 dummy wafers.
However, since the plasma used to clean the etching chamber in the dry cleaning process is formed by gases having fluoric elements, several fluoric radicals stick to the inner wall of the etching chamber. The fluoric radicals sticking to the inner wall of the etching chamber react with the chlorine and hydrogen bromide HBr while the etching process is performed to etch the silicide layer or polysilicon layer, so that the uniformity of the wafer surface becomes worse and the CD bias is increased due to the shrinkage of the etching rate (ER) even affecting the etching selectivity between polysilicon and oxide. Incidentally, the dummy wafers used in the dry cleaning process and the etching-chamber recovery process leads to the consumption of the wafers.
Because the amount of the fluoric radicals is decreased and the etching environment is gradually restored while the etching process is performed, the quality of the first wafers etched is especially unstable after the dry cleaning process is performed. The etching result of the following wafers becomes more stable as the stability of the etching environment improves. Since the abnormal etching effect is relatively obvious when the etching process is performed on the first wafer after the seasoning process, the unstable etching effect is called the first wafer effect. Commonly, the problem of the first wafer effect occurs in the etching process for forming the gate electrode comprising a silicide layer and a conductive layer.
The invention provides a waferless seasoning process. By using the invention, wafer consumption and the cost are greatly reduced. Moreover, the first wafer effect can be avoided, the duration between two wet cleaning processes can be greatly extended and the throughput is enhanced.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a waferless seasoning process suitable for an etching chamber of an etching machine when the etching environment is so bad that an etching process cannot be performed. A dry cleaning process with a plasma formed by oxygen and hydrogen bromide is performed to restore the etching environment in the etching chamber.
The invention provides a waferless seasoning process suitable for an etching chamber of an etching machine when the etching environment is so bad that etching cannot be performed. A dry cleaning process with a first plasma formed by oxygen and sulfonium hexafluoride is performed. An etching-chamber restoration process with a second plasma formed by hydrogen is performed.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.